1. Field of the Invention
The present invention relates to a position measuring system having a scale, on which an incremental graduation with a graduation period is provided, an interpolation unit, a counter, a reference track and a module for comparing the scanning signal of the reference track with a reference signal.
2. Discussion of Related Art
A position measuring system of this type is known from German Patent Publication DE 35 26 206 A1. A measurement representation in the form of an incremental graduation and a reference track with a continuously transparent field and an immediately following continuously opaque field is provided on a scale. This reference track is scanned by a window of a scanning unit in order to obtain a continuously weak scanning signal in the transparent field and a continuously strong scanning signal in the opaque field and to distinguish the two fields from each other in this way.
This arrangement has the disadvantage that in the course of scanning the transition from the opaque to the transparent field, the scanning signal increases relatively slowly. To achieve a dependable differentiation between the two field sections by means of a scanning signal it is necessary that the difference between the continuously weak and strong scanning signals be as large as possible. In accordance with the prior art this can be achieved by employing a window of the scanning unit which is relatively long in the measuring direction.
However, the result of using a window relatively long in the measuring direction is that the transition area of the scanning signal from the low to the high signal level is relatively broad. It is not possible to clearly determine a reference position by means of the scanning signal, because of which a separate reference marker is required. It is also not possible to clearly distinguish the two field sections to the right and left of the reference position by means of the scanning signal resulting from scanning the reference track and the reference marker.
A further position measuring system is known from Austrian Patent Publication AT 396 840 B, wherein continuous transparent and opaque field sections are placed next to an incremental graduation on a scale for distinguishing between the right and the left section of a reference position. In one reference track the field sections are arranged in a sequence of transparent and opaque, and complementary thereto in a second reference track opaque and transparent. At the reference position both reference tracks are transparent. A scanning signal is generated from each of the two reference tracks, and the two scanning signals are switched so that they differ.
In this arrangement the resulting scanning signal also rises relatively slowly, and the two reference tracks increase the structural size of the position measuring system.
A photoelectric device for the mutual alignment of two objects is described in German Patent Publication DE 20 46 332 C3. Two parallel reference tracks are arranged on one of the two objects and each reference track is scanned by a pattern of a scanning unit. Each reference track consists of a transparent and an opaque continuous field. Each reference track is designed with a pattern of a series of transparent and opaque fields at the transition between the two fields. The pattern of the scanning unit is complementary to this pattern. In the course of the movement of the scanning unit from the opaque field of a reference track to the transparent field of this reference track a state is achieved in which a steep rise of the scanning signal takes place because several transparent fields of the reference track simultaneously change from a non-overlapping state with the transparent fields of the scanning unit to an overlapping state. However, it is disadvantageous that it is not possible to determine a clear reference position by means of this scanning signal, since equal signal levels are being generated at several locations in the transition area. In order to rule out this ambiguity, the two scanning signals of the two reference tracks are switched so that they differ. The difference signal only has one unambiguous steeply rising section. However, it is disadvantageous that the difference between the low and the high signal level is very small because of this. Furthermore, two reference tracks and two scanning windows are required for generating the difference signal, which increases the structural size. The two reference tracks are arranged vertically in respect to the measuring direction at a distance from each other, which has the result that the difference signal is considerably affected by tilting between the scanning unit and the support of the reference tracks. The steeper the signal rise of the difference signal, the more sensitive the device is to tilting, since even with small tilts the difference signal can become zero within a large area.
An absolute position measuring system with several code tracks is described in Japanese Patent Publication JP 5-15213 B2. Each code track consists of alternating opaque and transparent fields. A pattern of successive transparent and opaque fields is provided at the respective transition between two fields, which is scanned by a pattern of the scanning device in order to obtain as steep as possible a signal rise of the analog scanning signals. The analog scanning signals are digitized and an absolute code word is formed. In connection with an exemplary embodiment, an incremental track is arranged parallel with the code tracks. However, none of the transitions between the opaque and transparent fields are used for generating a reference signal which is associated with a counting signal derived from the incremental graduation. Also, no steps and means for achieving such a definite association are recited. Therefore the transition between two fields does not constitute a reference marker.